The present invention relates to the operation of a closed loop feedback system that automatically corrects for skew error in the playback of a television video tape recording, and more particularly, to such a feedback system that performs such a correction whether or not vertical synchronization information is present.
When playing back a video tape recording, the effective length of the tape can change. This is called "skew" and can be caused by varying head to tape air gaps or by playing back the tape on a different machine than was used to record the tape, which can have a slight tension and tolerance difference from the recording machine. In any case, there is a change in the effective physical length of the tape on playback, which results in a change in duration of the recorded television horizontal lines. A recorded tape may be played back on a helical type video playback machine and, the resulting video may be displayed on a video monitor. When a helical-scan playback machine is arranged to synchronize the tape-derived horizontal sync signal with a local standard reference sync signal during each vertical sync interval, the difference in duration between the recorded horizontal lines and that defined by the standard reference sync signal causes a progressive change in the time of occurrence of the tape-derived horizontal sync signal relative to the standard sync signal during the remainder of the vertical interval. When reviewed on a monitor the horizontal deflection of which is synchronized with the local standard sync signal, the displayed video may include the tape-derived horizontal sync signal, thereby causing an apparent offset in the starting (left) edge of the displayed video, and this offset becomes progressively greater near the bottom of the raster. The effect of the skew variation is partially corrected at the bottom of the picture, when the video tape signal is resynchronized to an external reference synchronization source using the vertical synchronizing pulse from the recording. Further, if one wishes to splice in a segment of a video information on the tape, the durations of the spliced horizontal lines may be different, causing a skew transient in the displayed picture about the spliced recording.
It is therefore desirable to correct for the effects of this varying tape length on a time continuous basis.